1. Field of the Invention
The present invention relates to an image forming apparatus, such as copier, printer, and facsimile, and specifically relates to an image forming apparatus which can print data on one side of the recording sheet and also can print data on both sides of the recording sheet by inverting the recording sheet.
2. Description of the Related Art
Most of recent image forming apparatus, such as copiers, printers, and facsimiles, can print image information not only on one side of the recording sheet (also referred to as transfer sheet or sheet) but also on both sides of the recording sheet by means of electrophotography.
In some cases, such an image forming apparatus includes, for example, a face-up paper discharging function for discharging paper with the printed surface face up and a face-down paper discharging function for discharging paper with the printed surface face down so that those functions can be selected and executed when discharging the printed recording sheet. Specifically, when image information is printed on one side of the recording sheet, thereby making it possible for the printing condition to be checked immediately.
Generally, in an electrophotographic image forming apparatus, an electrostatic latent image which has been formed on an image carrier based on the image information of a document is developed by toner to become a toner image, and then is transferred onto the recording sheet. The toner image which has been transferred on the recording sheet is heated and pressurized by the fixing unit to melt and fix onto the recording sheet, and then the recording sheet is discharged outside the apparatus.
The recording sheet which has just passed through the fixing unit is still hot, and in a conventional image forming apparatus, a cooling fan blows air to quickly cool the recording sheet. But in some cases, the paper is not cooled fast enough resulting in incomplete fixation of the toner that has been melted and transferred onto the recording sheet.
Specifically when the face-down paper discharging operation or the inverting and conveying operation is to be conducted, the printed surface of the recording sheet comes in contact with members, such as rollers, located along the conveying path, and a portion of the toner image that has not been completely solidified and fixed is sticky, causing the recording sheet to stick to the rollers or to be contaminated by friction. Also, a phenomenon called “uneven cooling” occurs because difference in toner fixation arises between the portion of the toner image that has been immediately cooled, solidified, and fixed by coming in contact with the rollers and the other portion of the toner image where the toner has been slowly solidified and fixed. As a result, image roughness due to partially insufficient fixation occurs. Specifically, when the image is a color image, color hues greatly differ, causing the quality of the color image to greatly deteriorate.
Therefore, a conventional technology has been disclosed (for example, Japanese application patent laid-open publication No. 09-22228) which has a first cooling unit for cooling the image forming surface of the recording sheet that has been conveyed from the image forming section by blowing air inside the apparatus and a second cooling unit for cooling the backside of the recording sheet by blowing outer air directed from the outside of the apparatus so that the recording sheet which has just passed through the fixing unit can be quickly cooled from above and below by the first cooling unit and the second cooling unit respectively thereby preventing uneven cooling.
However, in the technology disclosed in the Japanese application patent laid-open publication No. 09-22228, the large temperature difference between the upper and lower air flow that is blown onto the recording sheet causes the recording sheet to greatly curl, and many malfunctions occur during the conveyance of the recording sheet on its way to being discharged. Therefore, complicated controls are necessary to make fine adjustments of the air temperature by detecting the outer air temperature, and stopping or operating the second cooling unit.
Furthermore, recording sheet was cooled by the air blown from the fan via an air duct at the air-blowing outlet immediately before the recording sheet is discharged from the apparatus body. Therefore, if the face-down paper discharging operation is conducted, the hot recording sheet which has just passed through the fixing unit comes in contact with conveying rollers and guide plates on the inverting path, causing uneven cooling.
Moreover, in some cases, an image forming apparatus that functions as a recording apparatus is connected to a sheet finisher (also referred to as finisher). The sheet finisher has complicated conveying paths so as to respond to a variety of post-processing procedures, and its conveying speed is faster than that of the image forming apparatus body to reduce the time used for the post-processing procedures. Due to the complicated conveying paths and high-speed conveyance, the conveying roller's conveying power is large, thereby easily causing the image to be contaminated inside the finisher or the paper to be tucked on the finisher's paper discharge tray. Accordingly, when using a finisher, a very high cooling capability is required. However, if the power of blowing air is high, when a finisher is not used, a problem arises in that the ability to stack paper on the paper discharge tray is reduced.
In the light of the above-mentioned problems, an objective of the present invention is to provide a simply-structured, inexpensive image forming apparatus which can form high-quality images by properly cooling the recording sheet that has just passed through the fixing unit, thereby preventing the image roughness, curl, contamination and adhesion of the paper due to the uneven cooling of the recording sheet.